An electrophotographic photoreceptor may have various structures depending on the characteristics necessary or the electrophotographic processes employed.
A system in which a photoreceptor comprising a support having thereon at least one photoconductive layer and, if necessary, an electrically insulating layer on the surface thereof is widely employed. The photoreceptor composed of a support and at least one photoconductive layer is subjected to ordinary electrophotographic processing for image formation including charging, imagewise exposure, development and, if necessary, image transfer.
Electrophotographic photoreceptors have also been used widely as an offset printing plate precursor for direct printing plate making. In particular, a direct electrophotographic lithographic printing system has recently acquired a greater importance as a system providing hundreds to thousands of prints of high image quality.
Binders to be used in the photoconductive layer should per se have film-forming properties and the capability of dispersing photoconductive particles therein. Moreover, when formulated into a photoconductive layer, binders should exhibit satisfactory adhesion to a support. They are also required to have various electrostatic characteristics and image-forming properties, such that the photoconductive layer exhibits excellent electrostatic capacity, small dark decay and large light decay, hardly undergo fatigue before exposure, and maintain these characteristics in a stable manner against a change of humidity at the time of image formation.
Binder resins which have been conventionally used include silicone resins (see JP-B-34-6670) (the term "JP-B" as used herein refers to an "examined Japanese patent publication"), styrene-butadiene resins (see JP-B-35-1960), alkyd resins, maleic acid resins and polyamides (see JP-B-35-11219), vinyl acetate resins (see JP-B-41-2425), vinyl acetate copolymer resins (see JP-B-41-2426), acrylic resins (see JP-B-35-11216), acrylic ester copolymer resins (see JP-B-35-11219, JP-B-36-8510 and JP-B-41-13946), etc. However, electrophotographic photosensitive materials using these known resins suffer from a number of disadvantages, such as (1) poor affinity for photoconductive particles (poor dispersion of a photoconductive coating composition); (2) low charging properties of the photoconductive layer; (3) poor quality of the reproduced image, particularly dot reproducibility or resolving power; and (4) susceptibility of the reproduced image quality to influences from the environment a the time of electrophotographic image formation, such as a high temperature and high humidity condition or a low temperature and low humidity condition; and the like.
To improve the electrostatic characteristics of a photoconductive layer, various proposals have hitherto been made. For example, it has been proposed to incorporate into a photoconductive layer a compound containing an aromatic ring or a furan ring containing a carboxyl group or a nitro group, either alone or in combination with a dicarboxylic acid anhydride as disclosed in JP-B42-6878 and JP-B-45-3073. However, the thus improved photosensitive materials still have insufficient electrostatic characteristics, particularly, light decay characteristics. The insufficient sensitivity of these photosensitive materials has been compensated for by incorporating a large quantity of a sensitizing dye into the photoconductive layer. However, photosensitive materials containing a large quantity of a sensitizing dye undergo a considerable deterioration in whiteness, which means reduced quality as a recording medium, and sometimes deterioration of dark decay characteristics occurs, resulting in the failure to obtain a satisfactory reproduced image.
On the other hand, JP-A-60-10254 (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application") suggests control of the average molecular weight of a resin to be used as a binder of the photoconductive layer. According to this suggestion, the combined use of an acrylic resin having an acid value of from 4 to 50 whose average molecular weight is distributed within two ranges, i.e., a range of from 1.times.10.sup.3 to 1.times.10.sup.4 and a range of from 1.times.10.sup.4 and 2.times.10.sup.5, would improve electrostatic characteristics, particularly reproducibility as a PPC photoreceptor on repeated use, moisture resistance and the like.
In the field of lithographic printing plate precursors using electrophotographic photoreceptor, extensive studies have been conducted to provide binder resins for a photoconductive layer having electrostatic characteristics compatible with printing characteristics. Examples of binder resins so far reported to be effective for oil desensitization of a photoconductive layer include a resin having a molecular weight of from 1.8.times.10.sup.4 to 10.times.10.sup.4 and a glass transition point of from 10.degree. C. to 80.degree. C. obtained by copolymerizing a (meth)acrylate monomer and a copolymerizable monomer in the presence of fumaric acid in combination with a copolymer of a (meth)acrylate monomer and a copolymerizable monomer other than fumaric acid as disclosed in JP-B-50-31011; a terpolymer containing a (meth)acrylic ester unit having a substituent having a carboxyl group at least 7 atoms distant from the ester linkage as disclosed in JP-A-5354027; a tetra- or pentapolymer containing an acrylic acid unit and a hydroxyethyl (meth)acrylate unit as disclosed in JP-A-54-20735 and JP-A-57-202544; a terpolymer containing a (meth)acrylic ester unit having an alkyl group having from 6 to 12 carbon atoms as a substituent and a vinyl monomer containing a carboxyl group as disclosed in JP-A-58-68046; and the like.
Nevertheless, these resins proposed have been proved by actual evaluations to be unsatisfactory for practical use in charging properties, dark charge retention, photosensitivity, and surface smoothness of the photoconductive layer.
The binder resins proposed for use in electrophotographic lithographic printing plate precursors were also proved by actual evaluations to give rise to problems relating to electrostatic characteristics, background staining of prints, and moisture resistance.
Further, known resins were found still to be insufficient to maintain performance properties in a stable manner regardless of considerable variations in environmental conditions of from high temperature and high humidity to low temperature and low humidity. In particular, an electrophotographic photoreceptor employed in a scanning exposure system using a semiconductor laser beam as a light source must possess higher electrostatic characteristic performance, particularly dark charge retention and photosensitivity, since the time of exposure is longer than that required in the case of conventional exposure to visible light over the entire surface thereof and also the exposure intensity is limited.